1. Field of the Invention
The invention relates to a microvalve, a microthruster system including the microvalve, both having application for spacecraft and other satellites, and in particular for use in orbital orientation, such as the 3-axis stabilization of microspacecraft, and to the methods of making and using such devices, and to satellites and other apparatus incorporating such devices. The disclosures of a co-filed application, entitled "Micro-Gas Rheostat" and of related subject matter, filed this same date, and the provisional application Ser. number 60/000,107 on which it is based, filed Jun. 9, 1995, are both incorporated herein by reference.
2. State of the Art
Spacecraft, including satellites (i.e., human-made satellites rather than natural satellites), both those carrying a living payload and those for such uses as extraterrestrial exploration, research, communications, surveillance, and the like, may typically have a number of different thrusters for propulsion and orientation of the craft. For present day satellites, which are of a relatively large construction, the thrusters are larger and more expensive than needed for microspacecraft. These traditional thrusters generally use solenoid-operated on-off valves and nozzles comprised of many different parts, with the parts individually made by traditional machine-shop cutting tools. Parts are individually assembled, using traditional attachment and sealing methods. There is presently a move towards developing smaller craft, such as microsatellites, which have reduced costs of both construction and launching.
Silicon is most familiar as the material used to make micro-size, inexpensive, multi-million element electronic microprocessors. The miniaturization, precision, low cost/high volume benefits of silicon batch processing have revolutionized the electronics industry. A derivative technology called "micromachining" allows precision micro-electro-mechanical systems (MEMS) to be inexpensively batch-processed from silicon, with precision, excellent producibility, and low-cost attributes analogous to such features of today's microelectronics.
The state of the art of miniature space application cold-gas thruster nozzles is perhaps best represented by thrusters recently produced for the Pluto Fast Flyby and Tethered Satellite System (TSS) programs. AIAA paper No. 94-3374 describes a miniature 0.001 lb.sub.f (0.004 Newton) nitrogen gas thruster produced by Moog, Inc., East Aurora, N.Y., for the Pluto Fast Flyby program. (See, e.g., AIAA Paper 94-3374, Miniature Propulsion Components for the Pluto Fast Flyby Space Craft, Douglas H. Morash, Moog, Inc. and Leon Strand, JPL, California Institute of Technology, Pasadena, Calif., presented at the AIAA Joint Propulsion Conference, Jun. 27-29, 1994, Indianapolis, Ind.) It uses a sliding-fit plunger type style solenoid valve and a conical diverging nozzle. The unit operates at 5 psia, weighs 0.016 lb (7.3 gram), requires 11 watts (28 vdc, 68.degree. F.), and responds in 0.94 ms (opening) and 0.2 ms (closing). The overall size is approximately 0.50 in. diameter by 1.00 long. Thruster performance (thrust level, specific impulse, impulse bit, etc.) was not measured in the referenced conference paper. Throat diameter is estimated to be 0.014 in.
Marotta Scientific Controls, Inc., Montville, N.J., with Devtec, Ltd., Dublin, Ireland, recently successfully developed and qualified a 0.225 lb.sub.f (1 Newton) nitrogen gas-thruster nozzle for the thruster of the Tethered Satellite System (TSS). The diverging section of the TSS nozzle has the classical "bell" shape. The TSS nitrogen gas thruster operates at 138 psia, has a 0.032 in. diameter throat and a 120:1 area ratio. The actual thruster Isp (specific impulse) was measured at 71 seconds.
It is generally agreed that a true microvalve is defined as one made, at least in part, of silicon material, and produced by micromachining, thus possessing precision, producibility, and miniaturization characteristics not possible with traditionally designed and fabricated valves. Using this definition, no microvalves have been used in the space program of any country. The state of the art of silicon microvalves is represented by commercial, non-flight products manufactured by three companies in the California "Silicon Valley" area. Commercially available microvalves are indeed much smaller and lighter than traditional valves, and major strides have been made in providing microvalves that "really shut-off", and in improving the manufacturing yield rates.
However, there is a major conceptual weakness common to these commercial microvalves when they are considered for space-flight applications in that they all employ thermally-based actuators (memory metal, thermal expansion bi-morph, and vapor pressure-type actuators). Thermally-based actuators impose several very severe product-application limitations - especially in the aerospace arena and high-end, non-flight commercial applications:
Limited Temperature Range: thermal (temperature-dependent) actuation severely limits the maximum temperature (to 50-60.degree. C.), as well as the temperature range, to values well under those needed in many aerospace applications. Thermal management will be an even greater problem in (and may even preclude) cryogenic usage.
Packaging Concerns: when several microvalves are closely manifolded together in the same silicon substrate, thermal management may preclude dense packaging (e.g., the heating of one valve may cause the actuation of an adjacent valve), and so thermally-actuated valves have a limited packaging density.
Unsuitable for Liquids: such thermally-based actuators suffer material incompatibility, the necessity to stay under the vapor temperature of liquid effluents, which is exacerbated by thermal conduction from the actuator to the effluent, and hence do not allow these devices to be used in most liquid, or corrosive gas, aerospace and medical applications and environments.
Low Sealing Force: these prior art devices typically generate a sealing force of about 5 to about 25 grams, as opposed to the instant devices which generate a sealing force on the order of 100 grams.
From a practical point of view, the state of the art in microvalves and microthrusters is a control valve that is essentially a hole in a piece of metal. As such, the tolerance of the hole diameter is about 15 mil (0.015 in.).+-.about 1 mil for accurate reproducibility. Further, for orientation thrusters, the supply gas tank is typically regulated in a single stage down to 300 psi, which must be further regulated down in a second stage to approximately 5 psia to achieve thrust forces on the order of one millipound (0.001 lb.sub.f). Thus, a complicated pressure control system is also required for the prior art apparatus.